Tire pressure monitoring systems (TPMS) have been known as systems that detect air pressure in car tires. Typically, a TPMS is mounted inside a tire and includes a sensor that detects air pressure and temperature, and a communication unit that wirelessly transmits data to a radio mounted on a vehicle. Therefore, the TPMS requires a power supply that supplies power to the sensor and other components.
Various power generating devices have been proposed as power supplies for use in TPMS etc. Particular attention has been focused on piezoelectric power generating devices using the piezoelectric effect. This is because piezoelectric power generating devices can be miniaturized relatively easily.
For example, as a piezoelectric power generating device for use in TPMS etc., Patent Document 1 discloses a piezoelectric power generating device that generates power in response to rotation of a tire by using periodic deformation of the tire. FIG. 12 is an exploded perspective view of a power generating device/sensor unit 110 disclosed in Patent Document 1. FIG. 13 is a vertical cross-sectional view of the power generating device/sensor unit 110 at rest. As illustrated in FIG. 12 and FIG. 13, in the power generating device/sensor unit 110, a piezoelectric element 111 is disposed inside a housing 112. The piezoelectric element 111 has a piezoceramic disc 114, on which an actuator 136 is disposed. When the tire rotates, centrifugal force acts on the actuator 136 and urges it against the piezoceramic disc 114. This causes the piezoceramic disc 114 to deflect. When an area of the tire adjacent to the power generating device/sensor unit 110 comes into contact with the ground, the centrifugal force experienced by the actuator 136 is reduced to substantially zero. This causes a reduction in urging force experienced by the piezoceramic disc 114 under the action of the actuator 136. Thus, in response to rotation of the tire, the centrifugal force applied to the actuator 136 periodically changes. As a result, the piezoceramic disc 114 vibrates and the power generating device/sensor unit 110 generates power.
Patent Document 2 discloses a power generating device 200 illustrated in FIG. 14. As illustrated in FIG. 14, the power generating device 200 includes a piezoelectric element 211a and a metal plate 211b bonded to the piezoelectric element 211a and serving as an extraction electrode. The piezoelectric element 211a and the metal plate 211b are supported by a fixed part 213 in a cantilevered manner.    Patent Document 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-501098    Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-49388
Since the piezoelectric power generating device disclosed in Patent Document 1 has a complex structure, it is difficult and expensive to manufacture this piezoelectric power generating device.
The power generator disclosed in Patent Document 2 has a structure simpler than that of the piezoelectric power generating device disclosed in Patent Document 1. However, the power generator disclosed in Patent Document 2 suffers from insufficient mechanical strength. Therefore, sufficient mechanical durability cannot be achieved in such applications as TPMS where relatively large force is applied to the piezoelectric element.